Technical Field
The present invention relates generally to gas turbine engine turbine segments having flanges attached to bands such as nozzle segments and shroud segments and, more specifically, chording of bands in such turbine segments shrouds.
Background Information
In a typical gas turbine engine, air is compressed in a compressor and mixed with fuel and ignited in a combustor for generating hot combustion gases. The gases flow downstream through a high pressure turbine (HPT) having one or more stages including one or more HPT turbine nozzles, shrouds, and rows of HPT rotor blades. The gases then flow to a low pressure turbine (LPT) which typically includes multi-stages with respective LPT turbine nozzles, shrouds, and LPT rotor blades. The HPT and LPT turbine nozzles include a plurality of circumferentially spaced apart stationary nozzle vanes extending radially between outer and inner bands. Typically, each nozzle vane is a hollow airfoil which cooling air is passed through. Cooling air for each vane can be fed through a single spoolie located radially outwardly of the outer band of the nozzle. In some vanes subjected to higher temperatures, such as the HPT vanes for example, an impingement baffle may be inserted in each hollow airfoil to supply cooling air to the airfoil.
The turbine rotor stage includes a plurality of circumferentially spaced apart rotor blades extending radially outwardly from a rotor disk. Turbine nozzles are located axially forward of a turbine rotor stage. The turbine shrouds are located radially outward from the tips of the turbine rotor blades so as to form a radial clearance between the rotor blades and the shrouds. The shrouds are held in position by shroud hangers which are supported by flanges engaging with annular casing flanges.
The turbine nozzles, shrouds, and shroud hangers are typically formed in arcuate segments. Each nozzle segment typically has two or more vanes joined between an outer band segment and an inner band segment. Each nozzle segment and shroud hanger segment is typically supported at its radially outer end by flanges attached to an annular outer and/or inner casing. Each vane has a cooled airfoil disposed between radially inner and outer band panels which form the inner and outer bands. In some designs, the airfoil, inner and outer band portions, flange portion, and intake duct are cast together such that the vane is a single casting. In some other designs, the vane airfoils are inserted in corresponding openings in the outer band and the inner band and brazed along interfaces to form the nozzle segment.
Turbine nozzles experience high stresses at the interface of the airfoil to the bands predominantly at the trail edge. The high stress results in cracking at these locations. One of the highest contributors to this stress is the chording which occurs on the bands due to the high temperature at the band flowpath combating the colder temperatures on the non-flowpath sides of the bands, particularly the flanges. Chording of the bands is bowing away from the flowpath. The chording associated with the bands imparts a stress at the airfoil band interface.
Certain two-stage turbines have a cantilevered second stage nozzle mounted and cantilevered from the outer band. There is little or no access between first and second stage rotor disks to secure the segment at the inner band. Typical second stage nozzle segments are configured with multiple airfoil or vane segments. Two vane designs, referred to as doublets, are a common design. Three vane designs, referred to as Triplets, are also used in some gas turbine engines. Doublets and Triplets offer performance advantages in reducing split-line leakage flow between vane segments. However, the longer chord length of the bands and mounting structure compromises the durability of the multiple vane nozzle segments. The longer chord length causes an increase of chording stresses due to the higher displacement of the longer chord length activated by the radial thermal gradient through the band. The increased thermal stress may reduce the durability of the turbine vane segment. Similarly, thermal stresses are present in turbine shroud segments and shroud hangers.
It is desirable to have turbine arcuate segments having flanges attached to bands that reduce chording and chording associated stresses. It is desirable to have turbine engine components such as the turbine nozzle arcuate segments and shroud arcuate segments having flanges attached to bands that reduce chording and chording associated stresses. It is desirable to have turbine engine components such as the turbine nozzle arcuate segments and shroud arcuate segments having flanges attached that reduce chording.